When resolving Domri’s first ability, if the card you look at isn’t a creature card, or if it’s a creature card you don’t want to put into your hand, you simply put it back on top of your library. You don’t reveal it or say why you’re putting it back.

3/14/2017

The second target of Domri’s second ability can be another creature you control, but it can’t be the same creature as the first target.

3/14/2017

If either target of Domri’s second ability is an illegal target when the ability tries to resolve, neither creature will deal or be dealt damage.

3/14/2017

If a creature has both double strike and trample, the combat damage it assigned during the first combat damage step will be considered when determining how much damage can trample through in the second combat damage step. If each creature blocking it is destroyed during the first combat damage step, then all of the damage must be assigned to the defending player or planeswalker during the second combat damage step.